1. Field of the Invention
The present invention relates generally to a gain and impedance switching circuit for an amplifier, and more particularly to a gain and impendance switching circuit for an amplifier which has practically no danger of oscillating during switching operation.
2. Description of the Prior Art
In the art, when cartridges that are different in output voltage and in output impedance are selectively used, provision has been made in the past that an amplifier is so switched that its input impedance and gain correspond to those of the cartridge. This switching of the amplifier is achieved in the art by a switching circuit such as shown in FIG. 1. In FIG. 1, 1 designates an input terminal, 2 and 3 changeover switches, 4 a pre-amplifier, 5 an equalizing amplifier, 6 the input terminal of an output amplifier, and 7 a feedback circuit.
When the circuit shown in FIG. 1 is connected to a moving magnet type cartridge (which will hereinafter be referred to as an MM cartridge), the change-over switches 2 and 3 are switched to their fixed contacts a, while when connected to a moving coil tape cartridge (which will be hereinafter referred to as a MC cartridge), the switches 2 and 3 are switched to their fixed contacts b, respectively.
In general, the MM cartridge has an output of 2 to 10 mV, and its input impedance is 50 K.OMEGA. as a standard load. While the MC cartridge has an output of 0.05 to 0.5 mV and its own output impedance is low or lower than 50.OMEGA., so that its input impedance is selected about 150.OMEGA.. If the input impedance of the MC cartridge is increased too high, a resonance is caused by the output impedance of the MC cartridge itself and the capacity of a cord connected thereto and hence there is danger that an oscillation is caused. For this reason, the input impedance of the MC cartridge is selected at most 150.OMEGA. generally.
Further, the pre-amplifier 4 has a flat frequency characteristic and the equalizing amplifier 5 has such a frequency characteristic that its output becomes low at a high band (RIAA) characteristic). The reason for this is that since a signal is recorded on a record with the signal level being high in a high band, the recording and reproducing characteristics are made average. Accordingly, in case of the MM cartridge, it is connected directly to the equalizing amplifier 5 through an input resistor R with the resistance value of 50 K.OMEGA., while in case of the MC cartridge, it is connected through an input resistor r with the resistance value of about 150.OMEGA. to the pre-amplifier 4 for the output therefrom to be amplified at about 20 to 30 dB and then to be applied to the equalizing amplifier 5. Therefore, with the prior art switching circuit shown in FIG. 1, the pre-amplifier 4 itself causes an increase in cost, and at the same time, since the input signal to the pre-amplifier 4 is high in level as the frequency becomes high, the tolerant input of the whole amplifier is limited by the maximum output of the pre-amplifier 4 at the high band.